Oven compensated crystal oscillators (OCXOs) are commonly found in analog and digital radio communication base stations to maintain a high frequency stability of typically ten parts per billion or less over a large temperature range. However, the operating frequency of a piezoelectric resonator will vary much more than this over temperature, generally on the order of twenty parts per million. To compensate, prior art OCXOs enclose a piezoelectric resonator within an oven to maintain the piezoelectric resonator within a small operating temperature range, no matter what the temperature outside of the oven.
Most prior art oven configurations have included a miniaturizing oven compartment, i.e. an insulated steel enclosed compartment with an enclosed resistive heating element, which surrounds a packaged piezoelectric resonator. These OCXOs are relatively large (several cubic inches) and draw a relatively large amount of current (100 mA).
Other prior art oven configurations have included a resistive heating element disposed directly onto the piezoelectric device itself, within the resonator packaging. Although this reduces the size of an OCXO, other problems arise. In particular, special crystal plating processes must be used, and the heating element adds contamination, adverse mass-loading, high thermal gradients, and stress to the piezoelectric device which results in undesirable frequency shifts and aging. Moreover, there are size constraints on the heating element as it must share space on the piezoelectric substrate with the existing electrodes.
The trend in radio base stations is towards more stable operating frequencies with smaller packaging and less power consumption without added costs due to special processing.
Therefore, there is a need for a heater apparatus for a piezoelectric device that does not require special processing or detract from the resonator performance or frequency stability. It would also be an improvement in the art if an OCXO could be provided that does not have a size or cost penalty and draws minimal current.